The exhaust gas emitted from an internal combustion engine is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”). The exhaust gas may also contain condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions typically disposed on catalyst supports or substrates (“catalyst devices”) are provided in the exhaust system of an internal combustion engine to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
Catalyst devices may be constructed of a variety of materials including ceramic monolith structures and metal monolith structures which are coated with catalyst compositions that are selected to treat specific exhaust gas constituents. Most catalyst devices are configured to operate within a given exhaust gas temperature range. Operation below the temperature range may result in underperformance of the catalyst device and operation above the temperature range may result in an operational life (i.e. durability) that is less than desired. Various engine control strategies have been proposed for controlling the operating temperature ranges of catalytic devices. Such control strategies have involved the injection of fuel into the exhaust system (direct injection of fuel or late fuel injection in the internal combustion engine) to either raise or lower the temperature of the exhaust gas. While effective, control strategies which utilize fuel for purposes other than extracting work from the internal combustion engine tend to negatively affect the overall fuel efficiency of the engine. Other control strategies involve the use of electrically heated devices such as grids placed on or in the catalytic devices for heating the device to a desired temperature range. Electrically heated solutions, while effective to heat the catalytic devices, utilize significant electrical energy which must be generated by the internal combustion engine and are not useful for purposes of lowering the operating temperatures of the catalytic devices.